The present invention relates both to the shape of the wire used in winding a speaker coil and to the electrical connections to the coil wire.
A conventional speaker has the configuration shown in FIG. 1. A voice coil 8 is provided in such a manner that its axis cuts the magnetic lines of force generated by a magnetic circuit. A periodic electric current flowing through the coil 8 causes it to vibrate parallel to a center pole 4 in accordance with Fleming s rule. The center pole 4 is placed within a magnet 7. The vibration of the voice coil 8 is transmitted through a bobbin 3 to a diaphragm 6 which radiates sound waves that propagate through the air.
The most commonly employed voice coil 8 in a speaker has the winding arrangement shown in FIG. 2 wherein a round insulated wire is wound around a bobbin 3 to form a coil. The winding arrangement employed in another conventional type of voice coil is shown in FIG. 3 wherein a ribbon of insulated wire is wound around the bobbin 3 in such a manner the individual turns stand on end with respect to the bobbin.
Ribbons of insulated wire can be fabricated by two methods. In one method, round wires with an insulation coating in place are flattened with a roll mill, and in the other method, conductors rolled to a flat form are then coated with insulation coating. Whichever method is employed, a conductor 1 with an insulation coating 2 is wound around the bobbin 3 and both ends of the insulated wire are soldered to a selected area of the diaphragm 6 for establishing electrical connection to a power supply. The coil 8 is positioned within a magnetic gap 9 (see FIG. 1) formed between the center pole 4 and a top plate 5 and, responsive to a signal current, the coil 8 vibrates to drive the diaphragm 6 which then radiates sound waves into the air.
As shown in FIG. 2, a round insulated wire is typically wound in two layers rather than in a single layer. In order to achieve connection of the coil 8 to the diaphragm 6 at two terminals, one terminal lead of the coil 8 is folded back into the magnetic gap 9 but this requires a corresponding increase in the width of the magnetic gap 9, causing a reduction in the force driving the coil to vibrate. In order to avoid this problem, the wire which has been wound up as a single-layered coil is wound on the second layer in the opposite pitch direction so that the two terminal leads of the coil can be lumped on one side. This allows the wire to be wound up as a double-layered coil, which is equivalent to the doubling of the number of coil turns. This is why the round insulated wire is typically wound as a coil in two layers.
On the other hand, an insulated ribbon wire is wound in a single layer as shown in FIG. 3, and this is because the ribbon is very thin and can be folded back into the magnetic gap without necessitating a substantial increase in the gap width.
The force F of driving the voice coil 8 is expressed by: EQU F=B.multidot.T.multidot.I.multidot.(N) . . . (1)
where B is the magnetic flux density generated by a magnetic circuit, T is the length of a conductor cutting the magnetic flux, and I is the current flowing through the voice coil. The term (N) is the number of turns of the conductor if T is calculated on a per turn basis.
In order to achieve greater vibration of the diaphragm in a speaker for a given amount of input current, the product of B and T must be increased without changing the weight of the voice coil.
The magnetic flux density B in a magnetic gap is expressed by: EQU B=(A.sub.m .multidot.B.sub.d)/(A.sub.g .multidot..delta.) . . . (2)
where A.sub.m is the cross-sectional area of a magnet, B.sub.d is the magnetic flux density at the operating point of the magnet, A.sub.g is the cross-sectional area of the gap, and .delta. is the coefficient of magnetic leakage. The calculation of magnetic flux densities is largely empirical, but it is generally understood that the smaller the width and cross-sectional area of the magnetic gap, the greater is the density of the magnetic flux that is generated in the gap.
Equation (1) shows that the coil driving force F increases with increasing length of the conductor which cuts the magnetic flux.
The force F which is necessary to drive two types of voice coils, the one made of a round wire as shown in FIG. 2 and the other made of a ribbon wire as shown in FIG. 3, can be estimated as follows. Cross sections of the two types of insulated wire are depicted in FIGS. 4 and 5. In order to evaluate the value of the force F, the size of the magnetic gap which varies with the type of wire from which a voice coil is made must be calculated.
If the coil shown in FIG. 3 is assumed to have the same total resistance, the same total width, and the same number of turns as the coil shown in FIG. 3, the thickness of the ribbon, T, shown in FIG. 5 is expressed by: EQU T=D/2+t . . . (3)
where t is thickness of the insulation coating and D is the diameter of the circular conductor. Since the cross-sectional area of the conductor is the same for both FIGS. 4 an 5, the widths of the two types of coil, L.sub.1 and L.sub.2, are calculated as follows: EQU L.sub.1 =2.multidot.(D+2t) . . . (4) EQU L.sub.2 =D.multidot.(3.multidot..pi.+4)/8+t . . . (5)
If the diameter D=0.22 and the insulation thickness t=0.005, the respective values of L.sub.1 and L.sub.2 are 0.46 and 0.374. Since the value of L.sub.1 is much greater than that of L.sub.2, this large value is a significant factor in the calculation of the size of the magnetic gap and contributes to a lower coil driving force.
Although L.sub.2 is smaller than L.sub.1, the actual value of L.sub.2 for the ribbon wire shown in FIG. 5 is so much greater than the idealized value that a satisfactory coil driving force is not attainable. As already mentioned, the manufacture of ribbon wires requires the rolling of round wires, irrespective of when this is effected before or after the application of an insulation coating. However, the rolling of round wires will inevitably introduce dimensional variances of L.sub.2, which must be absorbed by providing a sufficiently large magnetic gap. In addition, the rolled wire is not completely flattened and is somewhat oval. The heat conduction between adjacent turns of a voice coil made of an oval wire is not much larger than when the coil is made of a round wire, so that localized heat generation is inevitable when the voice coil made of a ribbon wire is heated. Therefore, in order to avoid deterioration of the insulator coating at locally heated portions, care must be taken to supply the voice coil of a ribbon wire with electric power which is not substantially greater than that applied to the voice coil made of a round wire.
The voice coil wound around a bobbin and a lead to an amplifier are usually connected with each other at a position above the bobbin, or on the side of the diaphragm to which the bobbin is mounted. In order to attain a large magnetic flux, the voice coil around the bobbin is usually multi-layered. If the wire is wound as a coil in two or any even number of layers, there is no problem in terminating the winding operation since both terminal ends are pulled out of the coil to become exposed at the same upward end of the bobbin. However, if the wire is wound in an odd number of layers, the end of the wire at which its winding is terminated is exposed on the side which is opposite the side where the other end of the wire is exposed (i.e., at which position the winding operation has been started). In this case, the terminating end of the wire must be directed to a position upward of the bobbin by guiding the terminating end to run over the voice coil, or guiding it to run between the bobbin and the voice coil. Otherwise, it may be partly bonded to the inner wall of the bobbin and the remaining free portion is pulled upwardly of the bobbin. In whichever method is used, however, the magnetic gap must be widened by an amount which corresponds to the diameter or thickness of the terminating end of the wire that is folded back with respect to the voice coil, and this results in an unavoidable drop in the coil driving efficiency.
Several techniques have been proposed to solve the aforementioned problems; one is described in Japanese Patent Publication No. 21158/67. The leads from the voice coil disclosed in this patent are hereunder described with reference to FIG. 6, wherein a voice coil 42 is supported on a bobbin 41 which is either metallic or made of paper with a surface metal foil. A terminating end 42 of the coil 42 is soldered to the lower part of the conductive bobbin 41, while the other end 42b of the coil 42 at which the winding operation has been started is directly connected to a lead 43. Another lead 44 is soldered to the conductive bobbin 41. Also shown are a diaphragm 45 and a damper 46.
In this coil arrangement, the bobbin 41 serves as part of the associated lead 44, and hence the aforementioned problem of an increased magnetic gap can be solved because there is no need to fold back the terminating end 42a of the voice coil 42. However, the method employed for joining the voice coil 42 to the bobbin 41 has the following disadvantages. If the voice coil 42 is made of an aluminum wire, as in the usual case, and is soldered to the bobbin 41, the device cannot be operated at temperatures higher than 200.degree. C. because the solder for joining aluminum wires melts at a temperature much below the point which the voice coil must withstand without failure (equal to or less than 300.degree. C.). Joining between an aluminum coil and a copper lead presents a problem with device reliability because of the corrosive attack of the flux remaining after aluminum soldering or due to the moisture-initiated electrolytic corrosion that is caused by the difference in ionization potential B between the metals present in the solder alloy (i.e., Zn, Sn and Pb).